Post-consumer polyethylene terephthalate (PET), which is primarily provided by or derived from plastic soft drink bottles, can be readily obtained from material recovery facilities. Such material exhibits relatively low and heterogeneous intrinsic viscosity (IV) values, and in the past, this characteristic has prevented PET from being directly used to produce products, such as, for example, high-performance plastic strapping, which in fact require relatively high and homogeneous intrinsic viscosity (IV) values. It was one of the discoveries of the invention disclosed within the aforenoted related U.S. patent application Ser. No. 08/794,538 that such heterogeneity of the intrinsic viscosity (IV) values of the PET material did not in fact adversely affect the production of high-performance strapping, and the present invention comprises a further improvement upon the processing of such PET material.
In accordance with prior art processing techniques, the PET material, whether post-consumer and/or non-post consumer material, was initially chopped into flakes and chunks, and the flakes and chunks were extruded into pellets. The chopped PET material had a relatively low and wide range of IV values because the various soft drink bottles, for example, were manufactured by different companies using different materials exhibiting different IV values. The IV values were typically within the range of 0.65-0.80 dl/g. In accordance with such prior art processing techniques, it was further believed that in order to make a high-performance product, such as, for example, high-performance plastic strapping, from such post-consumer PET materials, it was necessary that the materials exhibit or achieve a relatively high and narrow range of IV values after the solid state processing which therefore required, as an initial step, the pelletizing of the flakes before commencement of the solid state polymerization. When the PET pellets are then subjected to solid state polymerization (SSP), the pellets would have their IV values raised and exhibit a relatively high and narrow range of IV values whereby such enhanced pellets could then be used to produce high-performance products, such as, for example, high-performance strapping.
As noted hereinabove, in accordance with the noted prior art processing techniques, the prior art solid state polymerization (SSP) of the PET materials commenced with pellets of uniform geometry. Such prior art solid state polymerization (SSP) of the pellets, however, required an inordinate amount of time, that is, approximately twelve to nineteen hours, to complete in order to produce the desired strapping, and it was not appreciated, until the invention disclosed within the aforenoted U.S. patent application Ser. No. 08/794,538, that a heterogeneous mixture of flakes and chunk-like PET materials could undergo direct solid state polymerization, without necessarily being initially pelletized, to the same or higher average IV values as those of the prior art pellets, and in a significantly faster manner, that is, upon the order of one-quarter the time required for the solid state polymerization of the pellet materials.
More specifically, while the resulting prior art strapping exhibited average IV values which were not greater than 0.90 dl/g, high-performance plastic strapping fabricated in accordance with the processing techniques disclosed within the aforenoted U.S. patent application Ser. No. 08/794,538 exhibited average IV values which were greater than 0,90 dl/g. Therefore, in accordance with the teachings of the invention embodied within the aforenoted U.S. patent application Ser. No. 08/794,538, high-performance plastic strapping could be commercially manufactured in an economical manner using PET materials, having a relatively wide distribution of IV values, and as a result of undergoing solid state polymerization directly from flaked materials which do not have to be initially pelletized.
While the solid state polymerization processing of PET flake materials, and the production of the resulting high-performance plastic strapping, as disclosed within the aforenoted U.S. patent application, having Ser. No. 08/794,538, has been quite successful and has resulted in the production of highly suitable plastic strapping, it has been discovered that the process can be further improved from an efficiency and material flow-through production basis with decreased production downtime. For example, when the PET materials, which are to be used in accordance with the processing techniques of the aforenoted invention disclosed within U.S. patent application Ser. No. 08/794,538, are in fact derived from plastic soft drink bottles, it has been discovered that as a result of the blow-molding manufacturing techniques attendant the fabrication or manufacture of soft drink bottles, the recycled PET bottles comprise essentially two different types of materials, that is, substantially crystalline wall sections and substantially non-crystalline neck sections, and accordingly, such different materials must be handled or processed differently. More particularly, the wall sections are preferably to be retained and utilized within the solid state polymerization (SSP) process, while the neck sections are preferably to be discarded from the solid state polymerization (SSP) process in accordance with the following.
It is known, for example, that temperature control of the solid state polymerization (SSP) process is critical--if the temperature level of the solid state polymerization process is too low, the polymerization reaction will be too slow, while if the temperature level of the polymerization process is too high, the chips will melt or soften and thereby form clumps which will clog or jam the feeders or other components of the processing equipment. Since the wall portions of the recycled soft drink bottles are essentially crystalline, they react quickly within the solid state polymerization unit so as to desirably increase the molecular weight or intrinsic viscosity (IV) of the PET batch or charge, however, since the neck portions of the recycled soft drink bottles are substantially non-crystalline, they react quite slowly, if at all, within the solid state polymerization process in connection with the build-up or enhancement of the molecular weight or intrinsic viscosity properties of the PET materials being processed. Consequently, it is desirable from a processing efficiency point of view to have the batch or charge of PET materials within the solid state polymerization vessel to comprise more of the crystalline wall sections of the recycled PET materials than the non-crystalline neck sections of the recycled PET materials. In addition, and even more importantly, such non-crystalline neck portions or segments of the PET materials will crystallize rapidly within the polymerization processing vessel thereby emitting a significant amount of heat of crystallization. This heat of crystallization can be large enough to undesirably raise the temperature level of the process within the solid state polymerization vessel such that the PET materials tend to become sticky and agglomerate thereby forming clumps or chunks which will impede the flow of the materials within the polymerization vessel as well as clog or jam the various vessel components.
A need therefore exists in the art for effectively dealing with non-crystalline portions of recycled PET materials whereby, for example, such non-crystalline portions or segments of the recycled PET materials can be effectively removed from the batch or charge of PET materials to be fed into the solid state polymerization vessel such that processing or flow-through problems of the materials attendant temperature excursions, which would otherwise develop as a result of the rapid crystallization of such non-crystalline materials within the polymerization vessel and the consequent generation of a significant amount of heat of crystallization, would be obviated, and in addition, the processing efficiency of such recycled PET materials, and the resulting fabrication of high-performance strapping from such PET materials, can be effectively enhanced.